Modern complex electronic devices are rigorously tested during and after production. As electronic devices are implemented in ever smaller geometries, there is an increasing chance of failure over the life of a device due to changes resulting from changes in the materials. One well-known change resulting from device-material-drift is negative bias temperature instability (NBTI). Another is the “hot carrier effect”. Both can change device behavior during operations. Resistances of conductors in a device can also change over time due to electro-migration.
Several phenomena can reduce the reliability margin of a device. One is the total number of elements (e.g., transistors, interconnects, contacts, vias, etc.) in a device. If the probability of failure in an element remains relatively constant and thus the failure rate in time (FIT) per element stays constant, the overall FIT of the product will degrade. Another phenomenon reducing reliability is the higher, near-marginal, demand placed on each element to extract maximum performance as technologies advance. This increases the FIT rate of each element. Another phenomenon is the tightening of system reliability requirements.
Failures resulting from these phenomena are often only discovered later in the life of a system, long after manufacture and initial testing. Even very complex devices can survive a myriad of in-production and post-production tests but still harbor potential failures that aren't discovered until late in the device's life. Such failures can be detected after installation but usually require that the appliance or system in which a device operates is taken out of operation for testing.
When a system fails as a result of a failure of a complex semiconductor device in it, Built-in Self Test (BIST), usually exercised at power-up, can be used to locate the failed device for replacement. However, the locating and replacing do not take place until the system fails, incurring the cost and inconvenience of a system shutdown of the entire system or a board. The cost can also be in lost operating revenue and/or redundant functional boards and systems as back up for failures.
If a device is known to be developing a failure, its replacement can be planned and accommodated at a minimum impact to the system. However, predicting failure is not easy. One possible way to detect failure is to subject the device to a stress test in which the device is stressed beyond normal operation. Another way to detect failure is to collect electrical parameters (e.g., delays of selected paths) statistically under normal and/or stressed conditions across time and predict failure trend.
What is needed, then, is a means of subjecting a complex semiconductor device to high-stress test conditions and/or parameter measurements and identifying potential failures from the results of the stress testing and/or trends of parameters change. Such a means of testing should allow continued operation of the system in which the complex device is operating and it should be built into the device or the system.